Connecting plant–microbial interactions above and belowground: a fungal endophyte affects decomposition

Mutualisms can strongly affect the structure of communities, but their influence on ecosystem processes is not well resolved. Here we show that a plant–microbial mutualism affects the rate of leaf litter decomposition using the widespread interaction between tall fescue grass (Lolium arundinaceum) and the fungal endophyte Neotyphodium coenophialum. In grasses, fungal endophytes live symbiotically in the aboveground tissues, where the fungi gain protection and nutrients from their host and often protect host plants from biotic and abiotic stress. In a field experiment, decomposition rate depended on a complex interaction between the litter source (collected from endophyte-infected or endophyte-free plots), the decomposition microenvironment (endophyte-infected or endophyte-free plots), and the presence of mesoinvertebrates (manipulated by the mesh size of litter bags). Over all treatments, decomposition was slower for endophyte-infected fescue litter than for endophyte-free litter. When mesoinvertebrates were excluded using fine mesh and litter was placed in a microenvironment with the endophyte, the difference between endophyte-infected and endophyte-free litter was strongest. In the presence of mesoinvertebrates, endophyte-infected litter decomposed faster in microenvironments with the endophyte than in microenvironments lacking the endophyte, suggesting that plots differ in the detritivore assemblage. Indeed, the presence of the endophyte in plots shifted the composition of Collembola, with more Hypogastruridae in the presence of the endophyte and more Isotomidae in endophyte-free plots. In a separate outdoor pot experiment, we did not find strong effects of the litter source or the soil microbial/microinvertebrate community on decomposition, which may reflect differences between pot and field conditions or other differences in methodology. Our work is among the first to demonstrate an effect of plant–endophyte mutualisms on ecosystem processes under field conditions.     

Indirect interactions in the microbial world: specificities and similarities to plant–insect systems

Trophic interactions between bacteria, viruses, and protozoan predators play crucial roles in structuring aquatic microbial communities and regulating microbe-mediated ecosystem functions (biogeochemical processes). In this microbial food web, protozoan predators and viruses share bacteria as a common resource, and protozoan predators can kill viruses [intraguild predation (IGP)] and vice versa, even though these latter processes are probably of less importance. However, protozoan predators (IG predator) and viruses (IG prey) generally occur together in various environments, and this cannot be fully explained by the classic IGP models. In addition, controlled experiments have often demonstrated that protozoan predators have apparently positive effects on viral activity. These surprising patterns can be explained by indirect interactions between them via induced trait changes in bacterial assemblages, which can be compared with trait-mediated indirect interactions (TMIIs) in terrestrial plant–insect systems. Here, we review some trait changes in bacterial assemblages that may positively affect the activities and abundance of viruses. It has been suggested that in bacterial assemblages, protozoan predation may enhance growth conditions for individual bacteria and induce both phenotypic trait changes at the individual (e.g., filament-forming bacteria) and group level as a result of changes in bacterial community composition (e.g., species dominance). We discuss the specificities of aquatic microbial systems and attempt find functional similarities between aquatic microbial systems and terrestrial plant–insect systems with regard to TMII function.     

Competition and facilitation among plants for pollination: can pollinator abundance shift the plant–plant interactions?

We hypothesize interactions among plants for pollination may depend on pollinator abundance, which always varies among years and habitats and has different effects on plant reproductive success. Honeybee-pollinated plants, Lotus corniculatus, and its commonly coflowering neighbor, Potentilla reptans var. sericophylla, were used in a two-year project. We designed six types of plant combinations with different conspecific and interspecific flower densities in 2011 and repeated this in the same site in 2012. Meanwhile, we artificially increased pollinator abundance by hiring beehives only in 2011. Pollinator abundance as well as flower density significantly affected pollination of L. corniculatus plants from both the conspecific and interspecific plots. Total number of bees visiting a plot was enhanced by an increase in both the conspecific and interspecific flower densities regardless of high or low pollinator abundance. However, changes in visitation rates and fruit sets in the focal plants when flower densities were increased depended on pollinator abundance. Under high pollinator abundance, an increase in both the conspecific and interspecific flower densities significantly enhanced pollinator visits to L. corniculatus. However, under low pollinator abundance, the pollinator visitation rate remained unchanged as conspecific flower density increased, but decreased when there was an increase in interspecific flower density. Coflowering plants enhanced fruit sets of L. corniculatus only when the pollinator abundance was high. The findings suggest that the interactions among plants for pollination are influenced not only by a plant density threshold, but also by a pollinator abundance threshold.     

Diversity of plant–animal interactions: Possibilities for a new plant defense indicator value?

The interactions between herbivores and plants are of general interest in ecology. Even though the extensive research carried out during the last decades has culminated in many theories, additional studies are necessary to validate these findings. In particular, the hypotheses dealing with the complex interrelations of plant defense mechanisms and herbivores continue to be debated.In this paper, we develop a new indicator value that quantifies the defense mechanisms of Central European woody plants against large mammalian herbivores. The indicator value is based on three plant-specific traits: chemical defense (toxic compounds, digestion inhibitors), mechanical defense and leaf size. Our validation of the newly established indicator shows that evergreen woody plants have a significantly higher indicator value than deciduous woody plants. Moreover, plant defense is correlated with growth height: woody plants growing in the browsing zone preferred by large mammalian herbivores have significantly higher levels of defense compared with woody plants capable of growth high above the reach of large herbivores.We conclude that the new plant defense indicator value is a valuable tool for the validation of existing hypotheses and habitat calibration on a statistical basis. The quantification of plant mechanisms of defense against large herbivores produces a significantly better understanding of the multifaceted nature of plant–animal interactions and should contribute positively to future studies.     

Species-specific herbivore–herbivore interactions and plant responses to sequential attack by multiple herbivores on a perennial woody plant

When plants are sequentially attacked by multiple herbivores, herbivore identity and host specialization can greatly influence the patterns of herbivore–herbivore and plant–herbivore interactions. However, how prior herbivory and the resulting induced plant responses potentially affect subsequent herbivores deserves further investigation. In this study, we conducted a common-garden experiment that manipulated sequential herbivory by the specialist caterpillar Gadirtha fusca Pogue (Lepidoptera: Nolidae) and the generalist caterpillar Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae) on Chinese tallow, Triadica sebifera (L.) Small (Euphorbiaceae). We tested how prior exposure to herbivores with different levels of host specialization affected the performance of subsequently arriving con- and heterospecifics, as well as plant growth and defense responses under subsequent herbivory. We found that prior exposure to the specialist G. fusca facilitated the performance of subsequent conspecifics, resulting in a significant decrease in the growth (height and stem diameter at ground level) of tallow plants. However, prior exposure to the generalist S. litura did not affect the feeding of subsequent con- or heterospecifics or the growth of tallow plants. Sequential herbivory by specialist and generalist conspecifics resulted in lower levels of tannins and flavonoids, respectively, in leaves of tallow plants, whereas sequential herbivory by the two species did not affect the levels of tannins or flavonoids, compared to a single damage event. We conclude that herbivore species-specific plant responses appear to be more important than herbivore identity or specialization in determining herbivore–herbivore interactions and plant responses to sequential herbivore attack.     

Evaluation of different protocols for the analysis of lipophilic plant metabolites by gas chromatography–mass spectrometry using potato as a model

In the analysis of lipophilic plant metabolites by gas chromatography?Cmass spectrometry a step is required to release fatty acids and other analytes from complex molecules. Seven alternative methods were compared to the standard method of 1% H₂SO₄/50°C/16?h using Desirée and Phureja potato tubers as models. With two sodium methoxide alkali-catalysed methods (0.5?M NaOCH₃/50°C/1 and 16?h) recoveries of ferulic acids increased, long chain fatty acids and sterols decreased, 2-hydroxy acids were negligible, solanidine was absent and ??5-avenasterol isomerisation was minimal. Using a harsh alkali hydrolysis (1.0?M KOH/120°C/24?h) followed by a mild methylation (1% H₂SO₄/50°C/1.5?h), recoveries of polyunsaturated fatty acids were poor, sterols decreased but ??5-avenasterol isomerisation was minimal. With a mild alkali hydrolysis (0.5?M NaOH/100°C/5?min) followed by methylation with boron trifluoride (14%BF₃/100°C/30?min) recoveries of sterols and 2-hydroxy fatty acids were similar to the standard method and ??5-avenasterol isomerisation was high. Lower ferulic acid recoveries, absence of solanidine and overestimation of fatty alcohols were evident in both methods involving alkali hydrolysis. Three different methods using hydrochloric acid (1.00?M HCl/70°C/5?h, 0.63?M HCl/110°C/2?h and 2.00?M HCl/50°C/24?h) all gave increased recoveries of 2-hydroxy acids, ferulic acids, solanidine and sterols, although ??5-avenasterol isomerisation increased. Hydrochloric acid methods are recommended for studies requiring quantitative determinations (i.e. concentration of metabolite in sample). Either the hydrochloric acid methods or the standard sulphuric acid method are suggested for determining relative concentrations between samples, although there is a requirement for further studies.     

Buzz in Paris: flower production and plant–pollinator interactions in plants from contrasted urban and rural origins

Urbanisation, associated with habitat fragmentation, affects pollinator communities and insect foraging behaviour. These biotic changes are likely to select for modified traits in insect-pollinated plants from urban populations compared to rural populations. To test this hypothesis, we conducted an experiment involving four plant species commonly found in both urban and rural landscapes of the Île-de-France region (France): Cymbalaria muralis, Geranium robertianum, Geum urbanum and Prunella vulgaris. The four species were grown in four urban and four rural experimental sites in 2015. For each species and each experimental site, plants were grown from seeds collected in five urban and five rural locations. During flowering, we observed flower production and insect–flower interactions during 14 weeks and tested for the effects of experimental site location and plant origin on flower production and on the number of floral visits. The study species had various flower morphology and hence were visited by different floral visitors. The effect of experimental sites and seed origin also varied among study species. We found that (1) insect visits on P. vulgaris were more frequent in rural than in urban sites; (2) for C. muralis, the slope relating the number of pollinator visits to the number of flowers per individual was steeper in urban versus rural sites, suggesting a greater benefit in allocating resources to flower production in urban conditions; (3) as a likely consequence, C. muralis tended to produce more flowers in plants from urban versus rural origin.     

Performance of a filter loop reactor using Zymomonas mobilis and validation of the ethanol production model — II

A special loop recycle reactor with capillary crossflow filters was designed to enhance ethanol productivity. The new set-up did not comprise a classical reaction vessel with a stirrer but the working volume consisted only of the void volumes of the filters and the peripheral equipment, however, it behaved as a well mixed CSTR. A circulating pump provided for both mixing and recirculation of the culture fluid. Degassing was accomplished with a cyclone type device. Even if the circulating pump destroyed cells and automated backflushing of the filters could not prevent membrane fouling, maximum biomass concentration reached 98 g l⁻¹ at a dilution rate of 4 h⁻¹ and the maximum ethanol productivity achieved was 224 g l⁻¹ h⁻¹. Using the loop recycle reactor, the ethanol production model proposed in Part I of this study (Nipkow et al. (1986) J. Biotechnol. 3, 35–47) extended with a variable rate accounting for mechanical destruction of cells was verified. It was demonstrated that the predicted productivity of 350 g l⁻¹ h⁻¹ — exploiting the biological potential of Zymomonas mobilis — should be attainable if improved mechanical equipment in a filter recycle system is employed.     

Does long-term soil warming affect microbial element limitation? A test by short-term assays of microbial growth responses to labile C,N and P additions

Increasing global temperatures have been reported to accelerate soil carbon (C) cycling, but also to promote nitrogen (N) and phosphorus (P) dynamics in terrestrial ecosystems. However, warming can differentially affect ecosystem C, N and P dynamics, potentially intensifying elemental imbalances between soil resources, plants and soil microorganisms. Here, we investigated the effect of long-term soil warming on microbial resource limitation, based on measurements of microbial growth (¹⁸O incorporation into DNA) and respiration after C, N and P amendments. Soil samples were taken from two soil depths (0–10, 10–20 cm) in control and warmed (>14 years warming, +4°C) plots in the Achenkirch soil warming experiment. Soils were amended with combinations of glucose-C, inorganic/organic N and inorganic/organic P in a full factorial design, followed by incubation at their respective mean field temperatures for 24 h. Soil microbes were generally C-limited, exhibiting 1.8-fold to 8.8-fold increases in microbial growth upon C addition. Warming consistently caused soil microorganisms to shift from being predominately C limited to become C-P co-limited. This P limitation possibly was due to increased abiotic P immobilization in warmed soils. Microbes further showed stronger growth stimulation under combined glucose and inorganic nutrient amendments compared to organic nutrient additions. This may be related to a prolonged lag phase in organic N (glucosamine) mineralization and utilization compared to glucose. Soil respiration strongly positively responded to all kinds of glucose-C amendments, while responses of microbial growth were less pronounced in many of these treatments. This highlights that respiration–though easy and cheap to measure—is not a good substitute of growth when assessing microbial element limitation. Overall, we demonstrate a significant shift in microbial element limitation in warmed soils, from C to C-P co-limitation, with strong repercussions on the linkage between soil C, N and P cycles under long-term warming.     

Plant–plant interactions,environmental gradients and plant diversity: A global synthesis of community-level studies

Previous syntheses on the effects of environmental conditions on the outcome of plant–plant interactions summarize results from pairwise studies. However, the upscaling to the community-level of such studies is problematic because of the existence of multiple species assemblages and species-specific responses to both the environmental conditions and the presence of neighbors. We conducted the first global synthesis of community-level studies from harsh environments, which included data from 71 alpine and 137 dryland communities to: (i) test how important are facilitative interactions as a driver of community structure, (ii) evaluate whether we can predict the frequency of positive plant–plant interactions across differing environmental conditions and habitats, and (iii) assess whether thresholds in the response of plant–plant interactions to environmental gradients exists between “moderate” and “extreme” environments. We also used those community-level studies performed across gradients of at least three points to evaluate how the average environmental conditions, the length of the gradient studied, and the number of points sampled across such gradient affect the form and strength of the facilitation-environmental conditions relationship. Over 25% of the species present were more spatially associated to nurse plants than expected by chance in both alpine and dryland areas, illustrating the high importance of positive plant–plant interactions for the maintenance of plant diversity in these environments. Facilitative interactions were more frequent, and more related to environmental conditions, in alpine than in dryland areas, perhaps because drylands are generally characterized by a larger variety of environmental stress factors and plant functional traits. The frequency of facilitative interactions in alpine communities peaked at 1000 mm of annual rainfall, and globally decreased with elevation. The frequency of positive interactions in dryland communities decreased globally with water scarcity or temperature annual range. Positive facilitation-drought stress relationships are more likely in shorter regional gradients, but these relationships are obscured in regions with a greater species turnover or with complex environmental gradients. By showing the different climatic drivers and behaviors of plant–plant interactions in dryland and alpine areas, our results will improve predictions regarding the effect of facilitation on the assembly of plant communities and their response to changes in environmental conditions.     

Soil water uptake by trees using water stable isotopes (δ2H and δ18O)−a method test regarding soil moisture,texture and carbonate

Aims

Stable isotopes of oxygen and hydrogen are often used to determine plant water uptake depths. We investigated whether and to what extend soil moisture, clay content, and soil calcium carbonate influences the water isotopic composition.

Methods

In the laboratory, dried soil samples varying in clay content were rewetted with different amounts of water of known isotopic composition. Further, we removed soil carbonate from a subset of samples prior to rewetting. Water was extracted from samples via cryogenic vacuum extraction and analysed by mass spectrometry.

Results

The isotopic composition of extracted soil water was similarly depleted in both ¹⁸O and ²H with decreasing soil moisture and increasing clay and carbonate content. Soil carbonate changed the δ¹⁸O composition while δ²H was not affected.

Conclusions

Our results indicate that soil carbonate can cause artifacts for ¹⁸O isotopic composition of soil water. At low soil moisture and high carbonate content this could lead to conflicting results for δ¹⁸O and δ²H in plant water uptake studies.     

Inhibition of growth,and effects on nutrient uptake of arctic graminoids by leaf extracts — allelopathy or resource competition between plants and microbes?

Previous research has shown that plant extracts, e.g. from boreal dwarf shrubs and trees, can cause reduced growth of neighbouring plants: an effect known as allelopathy. To examine whether arctic and subarctic plants could also be affected by leaching of phytochemicals, we added extracts from the commonly occurring arctic dwarf shrubs Cassiope tetragona and Empetrum hermaphroditum, and from mountain birch, Betula pubescens ssp. tortuosa to three graminoid species, Carex bigelowii, Festuca vivipara and Luzula arcuata, grown in previously sterilized or non-sterilized arctic soils. The graminoids in non-sterilized soil grew more slowly than those in sterilized soil. Excised roots of the plants in non-sterilized soil had higher uptake rate of labelled P than those in sterilized soil, demonstrating larger nutrient deficiency. The difference in growth rate was probably caused by higher nutrient availability for plants in soils in which the microbial biomass was killed after soil sterilization. The dwarf shrub extracts contained low amounts of inorganic N and P and medium high amounts of carbohydrates. Betula extracts contained somewhat higher levels of N and much higher levels of P and carbohydrates. Addition of leaf extracts to the strongly nutrient limited graminoids in non-sterilized soil tended to reduce growth, whereas in the less nutrient limited sterilized soil it caused strong growth decline. Furthermore, the N and P uptake by excised roots of plants grown in both types of soil was high if extracts from the dwarf shrubs (with low P and N concentrations) had been added, whereas the P uptake declined but the N uptake increased after addition of the P-rich Betula extract. In contrast to the adverse extract effects on plants, soil microbial respiration and soil fungal biomass (ergosterol) was generally stimulated, most strongly after addition of the Betula extract. Although we cannot exclude the possibility that the reduced plant growth and the concomitant stimulation of microbial activity were caused by phytochemicals, we believe that this was more likely due to labile carbon in the extracts which stimulated microbial biomass and activity. As a result microbial uptake increased, thereby depleting the plant available pool of N and P, or, for the P-rich Betula extract, depleting soil inorganic N alone, to the extent of reducing plant growth. This chain of events is supported by the negative correlation between plant growth and sugar content in the three added extracts, and the positive correlation between microbial activity, fungal biomass production and sugar content, and are known reactions when labile carbon is added to nutrient deficient soils.     

Attract and deter: a dual role for pyrrolizidine alkaloids in plant–insect interactions

Pyrrolizidine alkaloids (PAs) are the major defense compounds of plants in the Senecio genus. Here I will review the effects of PAs in Senecio on the preference and performance of specialist and generalist insect herbivores. Specialist herbivores have evolved adaptation to PAs in their host plant. They can use the alkaloids as cue to find their host plant and often they sequester PAs for their own defense against predators. Generalists, on the other hand, can be deterred by PAs. PAs can also affect survival of generalist herbivores. Usually generalist insects avoid feeding on young Senecio leaves, which contain a high concentration of alkaloids. Structurally related PAs can differ in their effects on insect herbivores, some are more toxic than others. The differences in effects of PAs on specialist and generalists could lead to opposing selection on PAs, which may maintain the genetic diversity in PA concentration and composition in Senecio species.     

Microperoxidase 11: a model system for porphyrin networks and heme–protein interactions

We measured the circular dichroism (CD) and absorption spectra of the B-band region of microperoxidase 11 (MP11) as a function of temperature and peptide concentration. At micromolar concentrations, small MP11 dimers or trimers lead to excitonic coupling between low-spin and high-spin heme groups, to which the NH₂ group of the MP11 N-terminal and H₂O are bound as a sixth ligand, respectively. These aggregates convert into monomers with hexacoordinated high-spin heme groups with increasing temperature. This transition can be described by a two-state model. Aggregation becomes more extended at 50 μM concentration and causes some B-band hyperchromism, which reflects a J-type arrangement of heme groups linked together in the aggregates formed. At near-millimolar concentration, the CD and absorption spectra of the B-band region suggest the existence of even more extended and thermally stable aggregates, which might involve μ-oxo dimers of the heme groups. The degree of aggregation at 50 and 500 μM concentration increases substantially if the sample is freed from most of its oxygen in a N₂ atmosphere. The CD spectrum of the monomeric high-spin species is reminiscent of that observed for the unfolded alkaline conformation of the intact protein. Finally, we investigated the binding of acetylmethionine (AcM) ligands to the heme at aggregation-supporting conditions (500 μM concentration). The data suggest that the ligand prevents any substantial aggregation. As a surprising result, our data reveal that AcM–MP11 complexes exhibit a high-spin/low-spin mixture, with the high-spin configuration being stabilized at high temperatures.     

Synthetic alienation of microbial organisms by using genetic code engineering: Why and how?

The main goal of synthetic biology (SB) is the creation of biodiversity applicable for biotechnological needs, while xenobiology (XB) aims to expand the framework of natural chemistries with the non‐natural building blocks in living cells to accomplish artificial biodiversity. Protein and proteome engineering, which overcome limitation of the canonical amino acid repertoire of 20 (+2) prescribed by the genetic code by using non‐canonic amino acids (ncAAs), is one of the main focuses of XB research. Ideally, estranging the genetic code from its current form via systematic introduction of ncAAs should enable the development of bio‐containment mechanisms in synthetic cells potentially endowing them with a “genetic firewall” i.e. orthogonality which prevents genetic information transfer to natural systems. Despite rapid progress over the past two decades, it is not yet possible to completely alienate an organism that would use and maintain different genetic code associations permanently. In order to engineer robust bio‐contained life forms, the chemical logic behind the amino acid repertoire establishment should be considered. Starting from recent proposal of Hartman and Smith about the genetic code establishment in the RNA world, here the authors mapped possible biotechnological invasion points for engineering of bio‐contained synthetic cells equipped with non‐canonical functionalities.     

Post hoc assessment of host plant use in a generalist invader: implications for understanding insect–plant interactions and weed biocontrol

Structured host-choice and no-choice tests were conducted to help clarify the host plant interactions of an insect herbivore that is simultaneously seen as broadly polyphagous and pestiferous (in Africa) and host restricted/beneficial (in Australia). The research reported here involves specification of the host range of the invasive population of Scirtothrips aurantii found on Bryophyllum in Australia and included tests involving three separate lists of plant species considered to have the potential for thrips attack (plants of horticultural concern, native species at risk of attack and species listed for screening in the search for specialist B. delagoense biocontrol agents). This procedure was developed specifically to deal with the S. aurantii situation in Australia. Because the test species is already present in the field, the conclusions from the tests could be evaluated independently against field sampling results. Host testing revealed that the fundamental host range of the Bryophyllum population of S. aurantii includes Macadamia integrifolia, Mangifera indica and Kalanchoe blossfeldianna. However, the choice tests (involving B. delagoense) and a field survey of Man. indica demonstrated conclusively that the realised host range of S. aurantii in the field is restricted to Crassulaceae. We recommend that host testing of generalist insects not be discounted out of hand (for biological control) because of their perceived polyphagy. Any evidence of populations being strongly associated with the weed species of interest (through quantified host association studies in the native range) suggests further scrutiny of that population is warranted, by means of the host testing methods developed here and in conjunction with appropriate tests of the population’s species status.     

Evaluation of temperature and guanidine hydrochloride-induced protein–liposome interactions by using immobilized liposome chromatography

Hydrophobic interactions between nine model proteins and net-neutral lipid bilayer membranes (liposomes) under stress conditions were quantitatively examined by using immobilized liposome chromatography (ILC). Small or large unilamellar liposomes were composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and immobilized in a gel matrix by utilizing covalent coupling between amino-containing lipids and activated gel beads or avidin–biotin biospecific binding. Retardation of bovine carbonic anhydrase (CAB) in ILC was pronounced at particular temperatures (50 and 60 °C) where the local hydrophobicity of theses protein molecules becomes sufficiently large. Protein-induced leakage of a hydrophilic dye (calcein) from immobilized liposomes interior was also drastically enhanced at particular temperatures where large retardation was observed. For other proteins examined, similar results were also observed. The specific capacity factor of the proteins characteristic for the ILC and the amount of calcein released from immobilized liposomes were successfully expressed as a function of the product of the local hydrophobicities of proteins and liposomes, regardless of protein species and the type of the stress conditions applied (denaturant and heating). These findings indicate that lipid membranes have an ability to non-specifically recognize local hydrophobicities of proteins to form stress-mediated supramolecular assemblies with proteins, which may have potential applications in bioprocesses such as protein refolding and separation. ILC was thus found to be a very useful method for the quantitative detection of dynamic protein–liposome interactions triggered by stress conditions.     

Is the abundance of species determined by their functional traits? A new method with a test using plant communities

The relation between functional traits and abundance of species has the potential to provide evidence on the mechanisms that structure local ecological communities. The niche-limitation/limiting-similarity hypothesis, derived from MacArthur and Levins’ original concept, predicts that species that are similar to others in terms of functional traits will suffer greater competition and hence be less abundant. On the other hand, the environment-filtering/habitat-optimum hypothesis predicts that groups of species with functional traits that are close to the optimum for that environment, and are therefore similar to other species, will be more abundant. We propose a new niche-assembly model for predicting the relative abundance of species in communities from their functional traits, which can detect the patterns that would be expected from either of these hypotheses. The model was fitted to eight plant communities sampled in the Lake Ohau district of New Zealand. For seven of the sites, the patterns could not be distinguished from that expected under a null model. However, in one site there was highly significant departure from the null model in the direction expected from the niche-limitation hypothesis. The site was probably the most productive of those examined. It is possible that competition for light rather than belowground resources, or faster recovery from disturbance, allowed greater predictability. Surprisingly, the predictability was seen when just the presences of a species’ neighbours in trait space were taken into account, but not when the potential effects of those neighbours were weighted by their abundance. For three of the four model types, the effects of species on each other were consistently negative: a significant trend. These results contradict the various neutral models of ecological communities.